The present invention relates to a resin composition for cationic electrocoating which contains a catalyst metal in an acetylide form, to a process for producing the same, and to a cationic electrocoating composition.
Cationic electrocoating can provide substrates or coating target articles with coatings every nook and cranny even when said substrates are complicated in form or shape and can be performed automatically and continuously. Therefore, it is in general use as a technique of forming an undercoat on substrates, such as automotive bodies, which have a large size and are complicated in form or shape and are required to be highly resistant to corrosion. Furthermore, said technique is very highly efficient from the coating composition consumption viewpoint, hence economical, as compared with other methods of coating and, therefore, it has come into wide use as an industrial method of coating.
Cationic electrocoating is carried out by immersing a substrate, which is to serve as a cathode, in a cationic electrocoating composition and applying a voltage. The deposition of a coating layer on the substrate involves electrochemical reaction. and the coating layer that has deposited on the substrate surface upon voltage application has an insulating property. Therefore, with the progress of deposition of the coating in the process of coating, hence with the increase in deposit layer thickness, the electric resistance of the coating layer increases in proportion to the increase in layer thickness. As a result, the deposition of the coating composition at that site decreases and, instead, the deposition of a coating layer on some other uncoated site begins. In this way, the deposition of emulsion particles of the coating composition takes place successively from uncoated site to uncoated site. In the present specification, such successive formation of a coating on uncoated sites of a substrate is referred to as xe2x80x9cthrowing powerxe2x80x9d.
Such cationic electrocoating is generally used in undercoat formation, mainly for the purpose of rust prevention and, therefore, it is required that even substrates having a complicated structure should be provided with a coating having a certain predetermined film thickness at any part thereof. For this reason, sufficient levels of throwing power are to be secured.
As a cationic electrocoating composition excellent in throwing power, WO 98/03701 for instance discloses a cationic electrocoating composition comprising a basic resin containing a triple bond such as an ethynyl or nitrile group within its molecule.
However, for effecting such cationic electrocoating so as to secure the desired performance characteristics, bath control is important. If bath control is neglected, the desired performance characteristics cannot be secured. Therefore, from the productivity viewpoint, it is very important that coating composition preparation and bath control are easy to perform while sufficient levels of throwing power are secured.
Meanwhile, it is a general practice to use a metal compound as a curing catalyst and, thus, incorporate an organic transition metal complex or an organic acid salt of a transition metal, in such cationic electrocoating compositions. However, when an organic acid transition metal salt is used, the organic acid appears as an anion in the electrolyte bath. To avoid its adverse effect, bath control must be carried out strictly and this results in an increased burden of control. In electrocoating, bath control is generally operated by ultrafiltration. Part of the metal compound added as a catalyst is removed by this ultrafiltration, so that it becomes necessary to adjust and maintain the metal ion concentration in the bath to and at an adequate level.
Furthermore, in cases where an organic acid transition metal salt is used, it is necessary to design the cationic electrocoating composition taking the anion content into consideration and the degree of freedom in the designing is restricted. As regards organic transition metal complexes, they are generally hard to dissolve or disperse in organic solvents or water, offering a problem in that it becomes difficult to prepare coating compositions.
Meanwhile, the cationic electrocoating composition disclosed in the above-cited publication WO 98/03701 is shown to give desired performance characteristics under baking conditions of 180xc2x0 C.xc3x9720 minutes. However, recent years have given much importance to reductions in energy cost in the coating line, hence reductions in total energy in the production process, not only from the economical viewpoint but also from the viewpoint of environmental protection. For these reasons, it is desired that the electrocoating compositions be rendered polyfunctional so that the curability may be improved or the compositions may be designed more freely. In particular, it is desired that a cationic electrocoating composition which is excellent in throwing power and with which excellent curability can be realized under mild conditions be developed.
In view of the state of the art mentioned above, the present invention has one object to provide a resin composition for cationic electrocoating, a process for producing the same and a cationic electrocoating composition, with or by which the difficulties in bath control and coating composition preparation in the case of a metal compound being added as a curing catalyst can be solved and cationic electrocoating can be carried out with ease.
The present invention has another object to provide a resin composition for cationic electrocoating, a process for producing the same and a cationic electrocoating composition, with or by which a high level of throwing power can be realized and excellent curability and a sufficient level of curing can be attained even under mild baking/curing conditions.
According to its first aspect, the present invention provides an acetylide-form propargyl-containing resin composition for cationic electrocoating, which comprises a resin having a skeleton of epoxy resin and has a number average molecular weight of 500 to 20,000, and which contains, per 100 grams of the resin composition solids, 5 to 400 millimoles of sulfonium group and 10 to 495 millimoles of propargyl group, the total content of the sulfonium and propargyl groups being not more than 500 millimoles per 100 grams of the resin composition solids, said propargyl group being partly in the form of an acetylide.
According to its second aspect, the present invention provides a resin composition for cationic electrocoating, which comprises a resin having a skeleton of epoxy resin and has a number average molecular weight of 500 to 20,000, and which contains, per 100 grams of the resin composition solids, 5 to 400 millimoles of sulfonium group , 10 to 485 millimoles of propargyl group and 10 to 485 millimoles of unsaturated double bond, the total content of the sulfonium and propargyl groups and the unsaturated double bond being not more than 500 millimoles per 100 grams of the resin composition solids, said propargyl group being partly in the form of an acetylide.
According to its third aspect, the present invention provides a process for producing an acetylide-form propargyl-containing resin composition for cationic electrocoating, which comprises the steps of:
(1) reacting an epoxy resin having at least two epoxy groups per molecule with a compound (A) having a functional group capable of reacting with epoxy group and having propargyl group, to obtain a propargyl-containing epoxy resin composition;
(2) reacting the propargyl-containing epoxy resin composition obtained in step (1) with a metal compound to convert part of the propargyl group in said epoxy resin composition into its acetylide form; and
(3) reacting the remaining epoxy groups in the epoxy resin composition containing part of the propargyl group in the acetylide form as obtained in step (2) with a sulfide/acid mixture to thereby introduce sulfonium group thereinto.
According to its forth aspect, the present invention provides a cationic electrocoating composition which comprises the above-mentioned resin composition for cationic electrocoating.